Demodulation of an amplitude-modulated sensor signal from, for instance transducers like a Pressductor.COPYRGT. or a LVDT (Linear Variable Differential Transformer), is normally realized using analog techniques and implemented by phase sensitive rectification followed by low-pass filtering. The low-pass filtering is necessary for removing frequency components of higher order (at the excitation frequency and around its overtones) which are introduced by the phase sensitive rectification. In most applications today there is a demand that the signal processing should deliver a digital signal output and be able to communicate with a parent control computer. In many cases the sensor is part of a regulation system.
Such a regulation system is normally accomplished by including a microprocessor in the transducer electronic circuitry. Instead of having the usual analogue demodulation there are many advantages in also introducing digital techniques within this processing stage. The electronic device will for instance contain less components, be more easy to miniaturize and therefore less costly to produce. Besides, the same hardware may be used for several excitation frequencies as the necessary low-pass filtering then is made by means of software control
The trivial realization of the analogue solution by means of digital techniques according to the state of the art consists of sampling the transducer signal with a high sampling frequency, rectifying the signal in a phase sensitive way and then achieving low-pass filtering by using a digital filter. Due to the high sampling frequency this method first of all demands a high calculation capacity which must be implemented with a fast digital signal processor (DSP).
A phase sensitive rectification of the periodic quantity to be measured will normally be realized utilizing a phase-locked sampling of the periodic quantity. An example of such a sampling solution is for instance disclosed in U.S. Pat. No. 4,646,004 by the assignee of the present invention.
Another U.S. Pat. No. 5,055,843 discloses a sigma delta modulator with distributed pre-filtering and feedback. An additional filter used allows control of the quantization noise transfer function profile independent of the forward signal transfer function. However, this technical solution involves quite a complicated circuitry for the reduction of the quantization noise and does still not practically solve the technical problems discussed above.
Consequently, there is still a call for simplifying the processing of sensor signals from AC driven transducers to ensure a simple and low cost build-up as well as presenting a reliable operation offering a high measurement resolution. A technique considering an improved method as described below presents a new inventive solution to the problem.